Magneto-optical materials



Sept. 8, 1970 G. J. FAN ET AL 3,527,577

MAGNETO-OPT ICAL MATERIALS Filed May 5, 1968 2 SheetsQ-Shcet l FIG.1

F|.G 4A INVRENITORS BERNELL E. ARGYLE GEORGE J. FAN

THOMAS R. MC GUIRE MERRILL W. SHAFER Sept. 8, 1970 Filed May '5, 1968 FIG. 2.

ABSORPTION (cm G. J. FAN ET MAGNETO-OPTICAL MATERIALS 2 Sheets-Sheet 2 Rb Ni COx F3 WAVE NUMBER (1o cm") United States Patent O 3,527,577 MAGNETO-OPTICAL MATERIALS George J. Fan, Ossining, and Thomas R. McGuire and Merrill W. Shafer, Yorktown Heights, N.Y., and Bernell E. Argyle, North Haven, Conn., assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed May 3, 1968, Ser. No. 726,282 Int. Cl. C01g 53/08; G02f 1/22; H01f 1/00 US. Cl. 23-315 14 Claims ABSTRACT OF THE DISCLOSURE When part of the nickel of RbNiF is replaced by cobalt, manganese, iron, cadmium, zinc, strontium or cobalt mixed with calcium, or with calcium and iron, the hexagonal structure and ferromagnetic order of RbNiF are maintained, but the easy axis of magnetization is rotated from being perpendicular to the optic axis to being parallel to it. Such rotation allows the new composition to be highly suited as a magneto-optic modulator of light or as a memory element.

BACKGROUND OF THE INVENTION Previous work on the magnetic and optical properties of the ABF class of compounds (where A represents Na, K, Rb, Cs or T1) and B is a divalent magnetic transition metal ion (such as Ni, Mn, Fe, Co) showed that, in general, they crystallize with the cubic or pseudocubic perovskite-like structure and order antiferromagnetically.. However, certain writers found that RbNiF crystallized with a hexagonal type structure which was weakly ferromagnetic or ferrimagnetic. See pp. 261-287 of an article appearing in the Z. Anorg Chem., vol. 317 (1962) by W. Rudortf et al. Other related articles are by K. Lee et al. in the Physical Review, vol. 132, p. 144 (1963) and by W. W. Holloway et al. in the 1965 Physical Review Letters, vol. 15, p. 17.

Pure RbNiF is a hexagonal ferrimagnet which is highly transparent at visible wavelengths and shows high Faraday rotation. Consequently, such a material would normally lend itself to be used as a magneto-optical modulator or switch for visible light. However, a particularly undesirable feature of pure RbNiF is the fact that its easy axis of magnetization is perpendicular to its optic axis. Consequently, to obtain large Faraday rotations without the deleterious effects of natural birefringence, one has to apply magnetic fields as high as 20,000 oersteds and higher, or operate the modulator with a longitudinal Faraday effect so that, in the direction of the optic axis, there is a component of magnetization. In either case, the technical difficulties involved make pure RbNiF unattractive as a magneto-optical device.

SUMMARY OF THE INVENTION 3,527,577 Patented Sept. 8,, 1970 RbNiF other divalent transition elements, such as Mn, Fe and Cu, are capable of use as partial substitutes for the Ni in RbNiF While Ca is also desirable as a substitutional ion for Ni+ in RbNiF alkaline earth metals other than Ca, such as those from Group II-A and Group II-B of the Periodic Chart of the Elements; for example Mg, Sr, Zn, Cd, etc. can be partially substituted for nickel.

Thus it is an object of this invention to provide new compositions of the form of hexagonal RbNi A F that can be grown as high purity single crystals, or as high purity polycrystals, that are highly useful in light modulating devices, where A is either a divalent transition metal (21-29 in the Periodic Chart of the Elements) or an alkaline earth metal in Group II-A or II-B of the Periodic Chart of the Elements, or combinations of both.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

.* DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of an electro-optic system using the novel magneto-optical crystals of this invention.

FIG. 2 is a plot of the rotation of the plane of polarization for ditferent concentrations of Ni and Co in the hexagonal compound RbNi Co F FIG. 3 is a plot of absorption V. frequency for the compound RbNi ,,Co F for various concentrations of Ni and Co.

FIGS. 4A and 4B are drawings showing the effect of cobalt in concentration on the rotation of the easy axis ofmagnetization with respect to the optical axis of a hexagonal RbNi A 'F crystal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One example of a highly desirable compound of the form RbNi A,;F is RbNi Co F The latter is produced by taking 1 unit of purified RbF, 0.75 unit of NiF and 0.25 unit of 'CoF and grinding them together in a dry box to avoid contaminating the ingredients with the atmosphere. The ground particles, while still in the dry box, are pressed into pellets, placed into a platinum tube, the ends of which are pinched. The pinched tube is removed from the dry box and then sealed by fusing.

The platinum tube and its contents are put into a furnace and slowly brought from room temperature to 500-600 C., the heating process taking about 12 hours. Once the final temperature is reached, it is held at 500- 600 C. for four to five hours and then quickly raised to 750 C., at which temperature it is maintained for five to six hours, and then removed from the furnace to allow the platinum tube and its contents to gradually cool to room temperature. The final product, at this stage, is a polycrystalline powder. If desired, such polycrystalline powder can be made into pellets and heat treated to obtain polycrystalline RbNl0 75CO0 25F3 in that the latter is useful as a magneto-optic element.

The polycrystalline powder is removed from the platinum tube and placed in a platinum crucible, the latter being put into a furnace and taken quickly to 975 C. and then slowly cooled (6-8 hours) from 975 C. to about 775 C.; when the approximate temperature or 775 C. is reached, the crucible is gradually cooled (2-3 hours) to room temperature. At this stage, large single crystals are formed which are cut, polished and oriented for use as a magneto-optical element in a suitable light modulator device or system.

The compound RbNi Co F can also be produced by reacting polycrystalline RbNiF with polycrystalline RbCoF The polycrystalline powders are first prepared by reacting RbF with CoF or NiF with RbF. Then 0.75 RbNiF and 0.25 RbCoF are placed in a platinum crucible and treated in the manner discussed above in describing the first method of producing R'bN1o 5CO F to produce nbNi co r FIG. 2 shows the rotation (degrees/cm.) for various compositions of RbNi Co F The abscissa of FIG. 2 indicates the wave number or wave length of light that undergoes such rotation. It is seen that when there is no substitution of cobalt for nickel (x=0), the highest rotation (350 deg/cm.) takes place about 5000 A. At about 5880 A., it is less than 75 deg./cm., increasing to about 140 deg/cm. when x=.08 and is about 200 deg/cm. when x is equal to 0.25. FIG. 3 indicates that the bulk absorption of RbNi Co F also increases with increas ing cobalt concentrations; however such increases are more than offset by the sharp increase in the Faraday effect in the visible region.

Additionally, as seen in FIG. 4A, the easy axis of magnetization M, which in RbNiF is perpendicular (see M) to the optic axis of RbNiF is rotated so that for RbNi Co F it is now parallel to the optic axis O.A. It has been ascertained that for values of x .07 in the compound RbNi Co F the easy axis of magnetization is perpendicular to the optic axis, but for values x 0.l7, the easy axis of magnetization is parallel to the optic axis at 77 K. For values of x between 0.07 and 0.17, the easy axis of magnetization is at an angle to said optic axis. The Faraday rotation 0 (see FIG. 4B) for RbNi Co F is approximately 200 deg/cm. in the visible region, and a magnetic field +H or -H of only about 25 oersteds is needed to attain such 200 deg./ cm. rotation of plane polarized light L, whereas magnetic fields in excess of 20,000 oersteds were needed for the same rotation of pure R bNiF Thus when one looks along the optic axis (c-axis), the natural birefringence is no longer observed for compositions of x .17.

Other compounds that can be made in the manner described above for preparing RbNi Co -,F are For obtaining RbNi Ca F the mixture that is placed in the furnace and prepared in a way similar to the way RbNi -Co F is prepared is 1.0 RbF+0.75 NiF +0.25 CaF for obtaining RbN1u 75COu Ca0 15F 1.0 RbF is added to 0.75 NiF and 0.10 CoF and 0.15 CaF In generalizing the last formula to read changes in value of x from 0.5 to 0.30 and corresponding changes in the value of y produce a range of values in magnetic field of less than 25 to 40 oersteds needed to switch the sample of RbNi Ca Co F in the direction of the optic axis. These values are negligible compared to the 20,000 oersteds needed, at about 77 K., to switch a hexagonal RbNiF sample along the direction of the optical axis.

Other examples of substitution ions for Ni++ in RbNiF that have been made and tested are:

The various novel compounds were X-rayed and the following patterns were obtained. Table I is for the com pound. RbNig qcoo Fa, Table is for RbNi CO F Table IH is for RbNi Co F and Table IV is for RbNi Co Ca F The value of d is the measured distance in angstroms between planes in a crystal using conventional X-ray diffraction measuring techniques and I /I0 is the intensity of a diffracted X-ray beam from such planes measured in arbitrary units where 100 is the strongest intensity and other numbers are fractions of 100.

Tables V, VI and VII are magnetic data obtained by Well-known testing procedures for RbNi Co F Where 015x 03, RbNi Ca F where 0.55x 0.25 and RbNi Co Ca F for values of x and y, at different Curie temperatures where OgxgQSO and 053 0.28 and x+y 0. The term 0 indicates the magnetic moment of a compound in emu/gm, where emu is measured in. ergs/gauss.

X-ray data and magnetic data for Co Ca F TABLE I o.a o.7 3 d: I/Io TABLE II oJ os s d5 I/IO TABLE III o.a5 o.15 a d: 4.84 7 3.51 23 2.96 100 2.52 12 2.40 8 2.29 3 2.26 33 2.09 57 1.91 6 1.79 3 1.70 33 1.60 7 1.54 3 1.48 13 1.47 1.41 6 1.36 3

TABLE IV o.e5 o.2o o.15 a d: [/10 4.86 8 3 .52 22 2.97 100 2.54 14 2.43 10 2.31 5 2.30 40 2.13 65 1.94 6 1.82 5 1.74 40 1.64 5 1.59 5 1.53 1.52 1 10 TABLE v.RbNi1 ,O0,F1

v8.2 o (e TABLE V.RbNi1 ;a;Fa

08.2 ta K.) (emu/gm.)

TABLE VII.RbNi1.(x+ C0xCa Fs a: 11 t5 (e u/ The substitution of a divalent transition metal or a divalent alkaline earth metal, or both, for the Ni++ in single crystals of hexagonal RbNiF produces a highly desirable material that is particularly useful in magneto-optic derepresent the storage of a 1 and such storage state could be sensed by a large rotation of polarized light passing through the material. The storage of 0 could be representedby the sample magnetized in the opposite direction and could be sensed by an opposite rotation of polarized light, i-.e., the initial conditions set for the plane of polarization. As seen in FIG. 1, a crystal 2 of RbNi Co,,F or its equivalent, is located between spaced crossed polarizing filters such as polarizer 4 and analyzer 6. Crystal 2 is placed in a magnetic field produced by an electromagnet 8. A light source 10 and light detector unit 12 is so disposed that the detector 12 senses polarized light that is rotated by a magnetic field in the +M direction, but does not detect polarized light that is rotated by a magnetic field in the -M direction. The light from source 10 passes through openings 14 in the electromagnet 8 prior to entering and after passing through crystal 2.

A single crystal 2 of RbNi CO F in the shape of a parallelepiped 6 mm. long and 1 mm. in cross section was used as a light modulator up to frequencies of 100,000 cycles per second.

The specimen was cut so the optic or c-axis was along the long axis. It was mounted in an evacuated chamber in a holder cooled by liquid nitrogen (77 K.) and a 40 turn coil of #34 wire was wound around the sample. The coil was energized by a high frequency oscillator. Using the experimental set up shown in FIG. 1, where the electromagnet 8 is replaced by a coil, the light source 10 was monochromatic (5995 A.), and the detector 12 was a photomultiplier tube. Modulations from D.C. to 100,000 cycles per second were observed. At 100,000 cycles per second, a modulating field of i8 gauss peak to peak was used and about 10% of the sample was switched.

Although a frequency of 100,000 cycles per second was used to demonstrate the operation of these new materials as magneto-optical elements, it should be understood that such elements can be used at frequencies limited only by the switching speeds of the materials. Such speeds are of the order of nanoseconds or less.

These new compounds experience a drop in Curie temperature. For example, in the compound RbNi Co F the Curie temperature drops linearly from 136 K. when x=0 to K. when gc=0.25. In compounds such as RbNi Sr F RbNi Cd F one obtains a composition having a hexagonal structure whose magnetic and optical properties will vary. In general, Curie temperatures are all lowered in these compounds compared to their parent pure compounds, but to a lesser extent when magnetic ions c 5++, Mn++ and Fe++ are substituted for Ni++. Magnetic moments for the noted compounds increase for those that contain the substituted Ca++ and Cd++ ions for the Ni++ ions.

Additionally, another compound that has very desirable magneto-optical characteristics is where x varies from 0 to .50, y from 0 to .20 and z from 0 to .28.

The novel compounds cited above can be grown as single crystals having hexagonal structure and capable of Wide use as magneto-optical elements; such elements serving as light modulators, memory units, or capable of use wherever high rotations of polarized light are desired using relatively low magnetic fields for achieving such high rotations.

What is claimed is:

1. A composition of matter consisting of RbNi A F where A is selected from one of the divalent transition metals consisting of cobalt, manganese and iron and the values of x for the metals varies as follows: for cobalt, 0176x6050; for manganese, 0 x 0.50; and for iron, 0 x0.08.

2. A composition of matter consisting of RbNi A F where A is selected from one of the metals consisting of calcium, strontium, zinc, cadmium and chromium and the values of x vary as follows: for calcium, 0.05 x 025; for strontium, 0 x0.08; for zinc, 0 x0.15; for cadmium, 0 x0.20; and for chromium, 0 x0.55.

3. A hexagonal structured single crystal of where A is cobalt and 0.17x 0.50.

4. A hexagonal structured single crystal of where A is calcium and (1056x 025.

5. A hexagonal structured crystal of RbNi Co F Where O.17 xO,50.

6. A single crystal Of RbNi0 65CO0 20Ca-0 15F3- 7. A single crystal of RbNi Co Ca F where 0 x .50 and 0 y .28.

8. The compound RbNi Mn F where 0 x 0.50.

9. The compound RbNi Fe F where 0 x0.08.

10. The compound RbNi Cd F where 0 x0.20.

8 11. The compound RbNi Zn F where 0 x 0.15. 12. The compound RbNi Sr F where 0 x 0.08. 13. The compound RbNi C0 Fe Ca F where 0 x0.50, 0 y 0.20 and 0 z 0.28.

14. A magneto-optical element consisting essentially of a crystal Of RbNi0 5CO020Cao 5F3- References Cited Holloway et al.: Physical Review Letters, vol. 15 (1), July 1965, pp. 17-19.

Holloway et al.: Journal of Chemical Physics," vol. 45 (2), July 1966, pp. 639-643.

Boky et 211.: Solid State Communications, vol. 5, pp. 927-931 (1967).

HELEN M. MCCARTHY, Primary Examiner J. COOPER, Assistant Examiner US. Cl. X.R. 

